1. Field of the Invention
The present invention relates to a liquid crystal display device for use in a personal computer, a word processor, an amusement apparatus, a television, or the like, a method for producing such a display device, and a resist for use in such a method. More particularly, the present invention relates to a liquid crystal display device in which liquid crystal molecules are oriented in axial symmetry in each of liquid crystal regions which are partitioned from one another by a polymer wall and a method for producing such a display device.
2. Description of the Related Art
Conventionally, a nematic liquid crystal display device such as a TN (twisted nematic) or STN (super twisted nematic) liquid crystal display device has been known in the art as a display device for displaying images based on an electrooptical effect. Since, this type of liquid crystal display device has a limited viewing angle, considerable effort has been put forth in the art in order to increase the viewing angle.
For example, Japanese Laid-Open Publication No. 6-301015 and Japanese Laid-Open Publication No. 7-120728 disclose a so-called xe2x80x9cASM (Axially Symmetrically aligned Microcell) mode TN liquid crystal display device (hereinafter xe2x80x9cConventional Example 1xe2x80x9d) in which liquid crystal molecules are oriented in axial symmetry in each of liquid crystal regions which are partitioned from one another by a polymer wall. Typically, each liquid crystal region substantially surrounded by the polymer wall is corresponds to one pixel.
In the ASM mode liquid crystal display device, the polymer wall substantially surrounding the liquid crystal region is provided on a side of at least one of a pair of substrates facing a liquid crystal layer. In the presence of an applied voltage, the liquid crystal molecules in each liquid crystal region are oriented in axial symmetry, thereby reducing the viewing angle dependency.
An operation principle of this liquid crystal display device will be described below with reference to FIGS. 22A to 22D. FIG. 22A is a cross-sectional view illustrating the liquid crystal display device in the absence of an applied voltage, FIG. 22B illustrates polarization microscopy (in a crossed Nicols state) of the liquid crystal display device in the absence of an applied voltage, FIG. 22C is a cross-sectional view illustrating the liquid crystal display device in the presence of an applied voltage, and FIG. 22D illustrates polarization microscopy (in a crossed Nicols state) of the liquid crystal display device in the presence of an applied voltage.
The liquid crystal display device includes a pair of substrates 14 and 18, and a liquid crystal layer 16 interposed therebetween. The liquid crystal layer 16 includes liquid crystal molecules 11 having a negative dielectric anisotropy. Transparent electrodes 19 and 10 are provided on the substrates 14 and 18, respectively, on the side facing the liquid crystal layer 16. Vertical alignment films 21 and 22 are provided on the transparent electrodes 10 and 19, respectively. A tapered polymer wall 17 is provided on the side of the substrate 18 facing the liquid crystal layer 16. Apillar-like protrusion 20 is provided selectively on the tapered polymer wall 17. The tapered polymer wall 17 substantially defines a liquid crystal region 15. As will be described later with reference to FIG. 22C, the liquid crystal molecules 11 within each liquid crystal region 15 are oriented in axial symmetry about a central axis 12.
In the absence of an applied voltage, the liquid crystal molecules 11 are aligned in a direction substantially perpendicular to the substrates 14 and 18, as illustrated in FIG. 22A, by the anchoring force of the vertical alignment films 21 and 22. When observed by a polarization microscope in a crossed Nicols state, the liquid crystal region 15 in this state exhibits a dark field (normally black mode), as illustrated in FIG. 22B.
When a voltage is applied across the liquid crystal layer 16, a force acts upon the liquid crystal molecules 11 with a negative dielectric anisotropy and orients the molecules 11 so that the long axis of the molecules 11 is perpendicular to the direction of the electric field. As a result, the molecules 11 incline from a direction substantially perpendicular to the substrate, as illustrated in FIG. 22C (gray-level display state). When observed by a polarization microscope in a crossed Nicols state, the liquid crystal region 15 in this state exhibits an extinction pattern along the polarization axis, as illustrated in FIG. 22D.
As described above, the liquid crystal display device according to Conventional Example 1 operates in a normally black mode. In the normally black mode, the liquid crystal molecules 11 are oriented in a direction perpendicular to the substrate (thereby producing a black display) in the absence of an applied voltage, whereas the liquid crystal molecules 11 are oriented in axial symmetry about the central axis 12 formed for each liquid crystal region 15 (thereby producing a white display) in the presence of an applied voltage.
The term xe2x80x9caxially symmetrical orientationxe2x80x9d as used herein refers to an orientation of liquid crystal molecules where the liquid crystal molecules are oriented in a spiral pattern as illustrated in FIGS. 23B and 23C, for example, but also includes other orientations such as a concentric orientation or a radial orientation. Typically, the central axis for the axially symmetrical orientation substantially coincides with the direction normal to the substrate.
FIGS. 23A to 23C are schematic diagrams of a modeled liquid crystal region 15, illustrating an orientation of the liquid crystal molecules 11 in the liquid crystal region 15. FIG. 23A illustrates a plurality of liquid crystal regions 15 defined by the polymer wall 17, FIG. 23B illustrates an orientation of the liquid crystal molecules 11 in one liquid crystal region 15, and FIG. 23C illustrates the respective orientations of the liquid crystal molecules 11 in a top layer 15T, an intermediate layer 15M and a bottom layer 15B of the liquid crystal region 15.
With such an ASM mode axially symmetrical orientation, the viewing angle characteristic of the liquid crystal display device can be improved as follows.
In the TN mode, the liquid crystal molecules in each liquid crystal region are oriented in a single direction as illustrated in FIGS. 24D to 24F. When the liquid crystal display device in a gray-level display state, as illustrated in FIG. 24E, is viewed from directions A and B, the gray-level display is properly perceived only in one of the directions A and B, but not in the other.
On the contrary, in an axially symmetrical orientation, the liquid crystal molecules are oriented in two or more orientations as illustrated in FIGS. 24A to 24C. Thus, the apparent refractive index of the liquid crystal molecules as viewed from the direction A is averaged with that from the direction B, so that the light transmission from the direction A is substantially equal to that from the direction B, thereby realizing a desirable viewing angle characteristic even in a gray-level display state as illustrated in FIG. 24B.
As described above, in an ASM mode liquid crystal display device, the liquid crystal molecules are oriented in axial symmetry, so that there is little change in the contrast even when the observer changes its viewing direction, thereby realizing a wide viewing angle characteristic.
The ASM mode liquid crystal display device according to Conventional Example 1 may be produced through a polymerization-induced phase separation of a mixture containing a polymerizable material and a liquid crystal material.
A method for producing the liquid crystal display device according to Comparative Example 1 will be described below with reference to FIGS. 15A to 15I.
First, referring to FIG. 15A, a glass substrate 908 is provided (step a). Although not shown in FIG. 15A for the sake of simplicity, a color filter and an electrode are already formed on one side of the glass substrate 908. A method for producing a color filter will be described later.
Then, referring to FIG. 15B, a polymer wall 917 is formed in a matrix pattern, for example, on the side of the glass substrate 908 on which the electrode and the color filter are formed (step b). The polymer wall 917 is provided for orienting the liquid crystal molecules in axial symmetry. More specifically, the polymer walls 917 having a matrix pattern are formed by first spin-coating a photosensitive resin material on the glass substrate,908, exposing the material via a photomask having a predetermined pattern and then developing the exposed material. The photosensitive resin material may be of either a negative or positive type. Alternatively, the polymer walls 917 may be formed by using a non-photosensitive resin material with an additional step of providing a resist film as described below.
FIG. 28 illustrates a conventional dry film resist 30. The conventional dry film resist 30 includes a base film 31 (about 75 xcexcm thick) of polyethylene terephthalate to serve as a support. The dry film resist 30 further includes a cushion layer 32 (about 15 xcexcm thick) of a thermoplastic resin for improving the shape conformability (an ability to closely follow any unevenness existing on the surface of the object) for the thermo-compression bonding process of the film, an oxygen blocking film 33 (about 2 xcexcm thick) for preventing oxygen from binding to the resist thereby hampering the polymerization of the resist, a photosensitive resin layer 34 (about 2 xcexcm thick) to serve as the resist, and a cover film 35 (about 15 xcexcm thick) of polypropylene to serve as a resist protection film. The layers 32 to 35 are layered in this order on the base film 31.
A resist film can be provided by first peeling the cover film 35 off the dry film resist 30, pressing while heating the photosensitive resin layer 34 onto the substrate using a laminator, and then peeling and removing the base film (support) 31 off the cushion layer 32.
Then, referring to FIG. 15C, pillar-like protrusions 920 are patterned in a discrete pattern on a portion of the top surface of the polymer walls 917 (step c). The pillar-like protrusions 920 are also formed through exposure and development of a photosensitive resin material.
Then, referring to FIG. 15D, the polymer walls 917, the pillar-like protrusions 920 and the exposed surface of the glass substrate 908 are coated with a vertical alignment agent 921 of polyimide, or the like (step d).
Then, referring to FIG. 15E, a counter glass substrate 902 including an electrode (not shown) formed on one surface thereof is provided (step e).
Then, referring to FIG. 15F, the surface of the counter glass substrate 902 is coated with the vertical alignment agent 921 (step f).
Then, referring to FIG 15G, the substrates 908 and 902 are attached together so that their sides including the electrode formed thereon face each other, thereby producing a liquid crystal cell (step g). The interval between the two substrates (xe2x80x9cthe cell gapxe2x80x9d; the thickness of the liquid crystal layer) is determined by the sum of the height of the polymer wall 917 and that of the pillar-like protrusion 920. Thus, the thickness of the liquid crystal layer (the cell gap) can be adjusted to a desired thickness.
Then, referring to FIG. 15H, a liquid crystal material is injected into the gap in the liquid crystal cell using a vacuum injection method, or the like, thereby providing a liquid crystal region 915 (step h).
Then, referring to FIG. 15I, the liquid crystal molecules within the liquid crystal region 915 are oriented in axial symmetry by, for example, applying a voltage between the pair of electrodes provided on the pair of substrates (step i). Thus, the liquid crystal molecules within the liquid crystal region 915 partitioned by the polymer wall 917 are oriented in axial symmetry about an axis 918, which is denoted in FIG. 15I by a broken line vertically extending between the substrates 902 and 908.
FIG. 16 is a cross-sectional view illustrating a conventional color filter 1000. The color filter 1000 includes a glass substrate 1001 and a black matrix (BM) 1002 formed on the glass substrate 1001 for blocking light passing through a gap between adjacent colored portions. The color filter 1000 further includes red (R), green (G) and blue (B) colored resin layers 1003. Each set of R, G and B resin layers 1003 corresponds to one pixel. An overcoat (OC) layer 1004 (about 0.5 xcexcm to about 2.0 xcexcm thick) of an acrylic resin or an epoxy resin is provided to cover the glass substrate 1001, the black matrix 1002 and the colored resin layers 1003. The overcoat layer 1004 is provided for (1) improving the flatness of the color filter 1000 so that an ITO film to be deposited thereon will be continuous without any disconnection and (2) protecting the colored resin surface so that the etchant used in an ITO etching process will not etch the colored resin layer. A transparent electrode 1005 of an indium tin oxide (ITO) film is further formed on the overcoat layer 1004. While the BM film 1002 is typically a metal chromium film about 100 nm to about 150 nm thick, a non-metal material is also being used recently (e.g., a photoresist type material in which carbon particles are dispersed in an acrylic photosensitive resin). A material obtained by coloring a resin with a dye or a pigment may be used for the colored resin layer 1003, and the thickness of the colored resin layer 1003 is typically about 1 xcexcm to about 3 xcexcm.
The color filter 1000 as described above may be produced by first forming a photosensitive colored resin layer on a substrate and then. patterning the photosensitive colored resin by photolithography. For example, an RGB color filter can be produced by forming, exposing and developing a photosensitive colored resin layer three times using red (R), green (G) and blue (B) photosensitive resin materials, respectively. For example, the photosensitive colored resin layer may be formed by applying a liquid, in which a photosensitive colored resin material is diluted with a solvent, on a substrate by a spin-coating method, or the like, or by transferring a photosensitive colored resin material prepared in the form of a dry film onto a substrate. A color liquid crystal display device having a wide viewing angle characteristic can be obtained by producing the above-described ASM mode liquid crystal display device while using the color filter as described above.
However, the ASM mode liquid crystal display device according to Conventional Example 1 and the method for producing the same have the following problems. Where the pillar-like protrusions, which define the cell gap, are formed on the polymer wall by patterning a photoresist, the larger the liquid crystal display device is, the more difficult it is to form the pillar-like protrusions with a uniform height and a uniform shape across the liquid crystal panel. Thus, the cell gap may not be uniform across the panel, thereby reducing the display quality such as the brightness uniformity and the color uniformity. FIG. 17 illustrates an exemplary brightness variation (or brightness non-uniformity) which was observed in a conventional ASM mode liquid crystal display device. As illustrated in FIG. 17, a brightness variation of about xc2x15% or more occurred over a few centimeters along the panel, whereby a brightness variation at a pitch of a few centimeters was perceived even by human eyes. Moreover, the need to separately provide the pillar-like protrusions increases the number of steps required for the production process.
In order to increase the definition and the brightness of an ASM mode liquid crystal display device, it is desirable to reduce as much as possible the width and the height of the polymer walls, which are provided for orienting the liquid crystal molecules in axial symmetry. To do so, however, it is necessary to increase the relative height of the pillar-like protrusions, which defines the cell gap, with respect to the height of the polymer walls, thereby increasing the influence of the thickness variation which may occur during the formation of the pillar-like protrusions. In addition, it will be difficult to form the pillar-like protrusion within the top surface of a polymer wall, and the bottom surface of the pillar-like protrusion may extend beyond the top surface of the polymer wall.
Moreover, the step of forming the pillar-like protrusions reduces the production yield of the liquid crystal display device, and the photoresist used in the step is expensive, thereby increasing the cost of producing the liquid crystal display device.
The cell gap uniformity can considerably be increased by using spacer beads for defining the cell gap instead of the pillar-like protrusions of a photosensitive resin material. In particular, the spacer beads are first mixed with a liquid resist and applied on the substrate. The mixture is then exposed and developed, so that the cell gap is defined by the spacer beads being secured in the cured resist. However, this technique has a problem which will be described below with reference to FIG. 29.
Referring to FIG. 29, some of the spacer beads 43 secured in the cured resist film 51 may be lifted by a certain distance from a substrate 1. Thus, there may be a cell gap variation which can possibly be as large as the thickness of the cured resist film 52. In order to avoid this, it is necessary to, for example, perform an additional step for pressing down the spacer beads 43 to align them at a constant level before baking the resist.
Another method commonly used in the art is to disperse spacer beads during the production process. According to the method, however, the spacer beads may contaminate a part of the production line and cause problems among other production steps, possibly reducing the production yield.
According to one aspect of this invention, a liquid crystal display device includes: a pair of substrates; a liquid crystal layer interposed between the pair of substrates; and a polymer wall formed on one of the pair of substrates. A thickness of the liquid crystal layer is defined by spacer beads to be greater than a height of the polymer wall.
In one embodiment of the invention, the spacer beads are provided on the polymer wall.
In one embodiment of the invention, a top portion of the polymer wall is a flat region.
In one embodiment of the invention, the spacer beads are colored.
In one embodiment of the invention, the polymer wall is made of a photosensitive resin.
In one embodiment of the invention, the photosensitive resin is of a negative type.
In one embodiment of the invention, the polymer wall is made of a transparent material.
In one embodiment of the invention, the spacer beads are made of a transparent material.
In one embodiment of the invention, the spacer beads are colored.
In one embodiment of the invention, the spacer beads are secured while being partially buried in the polymer wall.
In one embodiment of the invention, the polymer wall includes a portion which is acute-angle-tapered or inclined with respect to the one of the substrates.
In one embodiment of the invention, an angle of the inclined portion is about 5xc2x0 to about 45xc2x0 with respect to the one of the substrates.
In one embodiment of the invention, the polymer wall is made of a photosensitive resin.
In one embodiment of the invention, the photosensitive resin is of a negative type.
In one embodiment of the invention, the polymer wall is made of a transparent material.
In one embodiment of the invention, the spacer beads are made of a transparent material.
In one embodiment of the invention, the spacer beads are colored.
In one embodiment of the invention, a width of the polymer wall is equal to or greater than about twice as much as a diameter of the spacer beads.
In one embodiment of the invention, the spacer beads are made of a transparent material.
In one embodiment of the invention, the spacer beads are provided in a region where there is no polymer wall.
In one embodiment of the invention, a top portion of the polymer wall is an inclined region.
In one embodiment of the invention, an inclination angle of the inclined region is about 10xc2x0 or more with respect to the substrate.
In one embodiment of the invention, the polymer wall is made of a photosensitive rein.
In one embodiment of the invention, the photosensitive resin is of a negative type.
In one embodiment of the invention, the polymer wall is made of a transparent material.
In one embodiment of the invention, the spacer beads are made of a transparent material.
In one embodiment of the invention, the spacer beads are colored.
In one embodiment of the invention, each of the spacer beads has an adhesive layer on a surface thereof.
Functions of the present invention having such a structure will be described below.
Conventionally, the pillar-like protrusions are formed on the polymer walls to define the thickness of the liquid crystal layer. According to the present invention, the thickness of the liquid crystal layer is defined by spacer beads. The spacer bead dispersion step, which is simpler and has a better production yield, can replace the conventional pillar-like protrusion formation step, which may reduce the production yield and increase the production cost. Thus, the liquid crystal display device of the present invention and the method for producing the same can improve. the production yield and reduce the production cost.
Moreover, according to the present invention, the conventional structure where the pillar-like protrusions are formed on the polymer walls to define the thickness of the liquid crystal layer is replaced with the structure employing spacer beads which have a better height uniformity. Thus, it is possible to provide a liquid crystal display device having a desirably uniform thickness across the liquid crystal layer and an improved display quality.
When the thickness of the liquid crystal layer is defined by the spacer beads placed on the polymer wall, it is possible to suppress the disturbance in the orientation of the liquid crystal molecules which may be caused by the spacer beads.
When the polymer wall includes a top portion which is a flat region, the spacer beads are more easily placed on the top of the polymer wall, whereby the thickness of the liquid crystal layer (the cell gap) can be more uniform, and it is possible to prevent the deterioration of the display quality which may occur due to a variation in the cell gap across the panel plane. In such a case, it is possible to employ a method in which the spacer beads can be dispersed across the entire surface of the substrate, thereby improving the production yield and reducing the production cost.
Alternatively, the cell gap may be defined by spacer beads which are secured while being partially buried in the polymer wall. In such a case, it is not necessary to provide the pillar-like protrusions or increase the height of the polymer wall. Therefore, it is possible to prevent the cell gap from varying due to a possible variation in the thickness of the applied photosensitive resin. Moreover, it is not necessary to increase the width of the polymer wall so that the bottom surface of the pillar-like protrusion does not extend beyond the top surface of the polymer wall, whereby it is possible to increase the definition and the brightness of the liquid crystal display device. Thus, it is possible to realize a liquid crystal display device having a desirable display quality and a wide viewing angle characteristic. Moreover, since the liquid crystal display device has a simplified structure, it is possible to inexpensively mass-produce the liquid crystal display device with a stable quality.
The spacer beads can be secured while being partially buried in the polymer wall, by first applying the spacer beads on a substrate while being mixed in a photosensitive resin material, and then patterning the applied mixture by photolithography. Thus, it is possible to place the spacer beads on the polymer walls with a high controllability. Therefore, the step of producing the pillar-like protrusions by photolithography, which is required in the conventional method, can be eliminated, thereby also eliminating the need to align the pillar-like protrusions with the polymer wall.
When the polymer wall includes a portion which is acute-angle-tapered or inclined with respect to the substrate, it is possible to suppress the disturbance in the orientation of the liquid crystal molecules located in the vicinity of the polymer wall (along the periphery of the liquid crystal region). The term xe2x80x9cacute-angle-taperedxe2x80x9d as used herein means that the portion is tapered so that the width at the top of the portion is smaller than the width at the bottom thereof. Thus, it is possible to prevent light leakage through the periphery of the liquid crystal region, thereby improving the contrast in a black display.
Particularly, when the inclination angle of the inclined portion is set to be about 5xc2x0 to about 45xc2x0, it is possible to realize a stable axially symmetrical orientation and thus to reduce the height of the polymer wall.
When a photosensitive resin (resist) is used for the polymer wall, it is possible to substantially eliminate the need to separately form a resist film for the patterning process and thus to reduce the number of steps required for the production process.
A portion of the photosensitive resin directly under a spacer bead may not be sufficiently exposed to light. Then, if a positive photosensitive resin is used for the polymer wall, the spacer bead may remain within the pixel aperture. Therefore, a negative photosensitive resin is preferably used.
When a transparent resin is used for the polymer wall, the orientation of the liquid crystal molecules existing on the polymer wall can contribute to a display, thereby considerably improving the brightness of the display. Although the liquid crystal molecules existing on the polymer wall are not oriented in axial symmetry, such an area is small and the liquid crystal molecules existing on the polymer wall are randomly oriented, whereby those liquid crystal molecules do not adversely influence the display as a whole. Moreover, by reducing the height of the polymer wall, it is possible to suppress the amount of light attenuated through the transparent polymer wall, thereby improving the light transmission of the display device.
If the width of the polymer wall is equal to or greater than about twice as much as the diameter of a spacer bead, it is possible to reliably place the spacer bead within the width of the polymer wall.
As described above, the liquid crystal display device of the present invention includes a pair of substrates opposing each other with a liquid crystal layer therebetween, and polymer walls provided on at least one of the substrates for dividing the liquid crystal layer into a plurality of liquid crystal regions. The thickness of the liquid crystal layer is defined by the spacer beads to be greater than the height of the polymer wall. The liquid crystal molecules in each liquid crystal region are oriented in axial symmetry about an axis perpendicular to the substrate. Thus, it is possible to increase the viewing angle of a liquid crystal display device while utilizing the ASM mode liquid crystal orientation.
When the thickness of the liquid crystal layer is defined by the spacer beads existing in a region where there is no polymer wall, it is possible to maintain a constant cell gap over a large area, while preventing the cell gap from varying even when there is a variation in the height of the polymer walls. Thus, it is possible to maintain the cell gap uniform across the panel plane and to prevent the display quality from deteriorating due to a variation in the cell gap.
The top portion of the polymer wall may be an inclined region. In such a case, assembling of the liquid crystal cell can be performed after a simple dispersion of the spacer beads across the entire surface of the panel. Then, any spacer beads existing on the polymer walls will move down along the inclined surface of the polymer wall into an aperture region. Thus, in a simple and reliable manner, it is possible to ensure that the thickness of the liquid crystal layer is defined by the spacer beads which exist in a region where there is no polymer wall. Therefore, it is possible to mass-produce the liquid crystal display device having the desirable effects as described above with a stable quality. It has been experimentally confirmed that the above-described effects are provided when the inclination angle of the inclination region is about 10xc2x0 or more with respect to the substrate.
It has also been experimentally confirmed that by the use of colored spacer beads (e.g., black) instead of using the transparent spacer beads, it is possible to suppress the disturbance in the axially symmetrical orientation due to the presence of the spacer beads within the liquid crystal region, and to prevent the display quality from deteriorating due to possible leakage of light passing through the spacer beads aggregated within the liquid crystal region.
The relationship between the display quality and the spacer bead dispersion density was studied for a liquid crystal display device using colored spacer beads. The spacer bead dispersion density was defined by the number of groups of spacer beads (each group including a few spacer beads which are aggregated together) in a given size of liquid crystal region. It was found that where colored spacer beads are used, the display quality does not substantially deteriorate even when the spacer bead dispersion density is as high as, for example, about 8-10 groups per liquid crystal region (having a size of about 100 xcexcmxc3x97100 xcexcm). Thus, with colored spacer beads, it is possible to increase the spacer bead dispersion density by about 2-fold to about 2.5-fold from that when transparent spacer beads are used, without substantially deteriorating the display quality. Thus, it is possible to considerably increase the process margin.
The above-described liquid crystal display device may be produced by a method including the steps of: coating a photosensitive material on a substrate: dispersing spacer beads on the substrate having the photosensitive material being applied thereon: and forming polymer walls by patterning through photolithography the photosensitive material on the substrate having the spacer beads being dispersed therein. Then, it is possible to provide the spacer beads selectively on the polymer walls. Thus, it is possible to prevent the disturbance in the orientation of the liquid crystal molecules which may be caused by the presence of the spacer beads within the pixel aperture, thereby further improving the display quality.
Alternatively, a spacer bead dispersion process and a heat treatment may be performed after exposing the polymer wall pattern (provided for orienting the liquid crystal molecules in axial symmetry), followed by a development process. In this way, it is possible to realize a structure where the spacer beads exist only on the polymer walls. Therefore, it is possible to reduce the contamination of the production line due to the spacer beads, thereby further improving the production yield.
The post-exposure heat treatment not only adjusts the sensitivity (resolution) of the photosensitive material for the polymer wall but also secures the spacer beads. Thus, no additional step is required.
Moreover, when the surface of the spacer beads is coated by an adhesive material which is suitable for the conditions under which the post-exposure baking process for the photosensitive material is performed, it is possible to more reliably secure the spacer beads on the polymer walls. Thus, it is possible to produce a high-quality liquid crystal display device with an even higher production yield while preventing the production yield or the display quality from deteriorating.
According to the described above production, a liquid crystal display device is produced by first coating a photosensitive transparent acrylic resin (the material for the polymer wall) on a substrate, dispersing the spacer beads for defining the cell gap, and then exposing and developing the applied resin using a mask having a predetermined polymer wall pattern, so as to leave the spacer beads selectively on the patterned polymer walls. In such a case, it is preferred to disperse the spacer beads prior to pre-baking the photosensitive resin so that the spacer beads are more likely to be secured on the surface of the resist. However, performing the spacer bead dispersion process before the pre-bake process means that the spacer bead dispersion process is performed between the coating process and the heat treatment. Then, the resist is in a half-dried state for a long period of time, whereby a foreign substance is more likely to attach to the resist, reducing the production yield. Moreover, since the substrate is carried into an exposure apparatus with the spacer beads being dispersed on the substrate, the exposure apparatus may be contaminated by the spacer beads. Particularly, in the production of a large liquid crystal display device for which a proximity exposure method (where the substrate and the photomask are brought into a close proximity to each other) is often used, the contamination of the photomask by the spacer beads may present a more serious problem.
In view of this, it is alternatively possible to disperse the spacer beads after applying and exposing the photosensitive material in the same manner as in the prior art. In this way, it is possible to prevent the spacer beads from contaminating the exposure apparatus or the production line used between the application step and the exposure step, and thus to prevent the production yield from lowering.
Alternatively, the development process may be performed after the spacer beads are fused onto the photosensitive material by using a PEB (post-exposure baking) step for-adjusting the sensitivity (resolution) of the photosensitive material. In this way, no additional step needs to be provided.
The spacer beads are more reliably secured on the polymer walls if an adhesive material, which melts at a temperature lower than the heat treatment temperature, is applied on the surface of, the spacer beads before the heat treatment.
Thus, the invention described herein makes possible the advantages of: (1) providing a liquid crystal display device having a simplified structure, a uniform cell gap, a good display quality and a wide viewing angle characteristic which can be produced with a reduced number of production steps and with a stable quality, and which can be mass-produced inexpensively; and (2) providing a method for producing such a liquid crystal display device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.